In recent years, various processes for producing cans and other welded containers have been developed. Conventional cans made by soldering a tin-plated steel sheet exhibit excellent quality and reliability and, therefore, are widely used for containing various materials. However, the conventional soldered cans are disadvantageous in the following items.
1. When a solder consisting of pure tin is used, the cost of the soldering procedure is high.
2. When a solder consisting of a lead-tin alloy is used in order to reduce the soldering cost or to enhance the solderability, sometimes a portion of the lead bleeds into the material contained in the can. This causes the contained material to become contaminated and, if the contained material is a food product, the product, when ingested, could be injurious to the health of that person.
3. Also, in the case where the tin-plated steel sheet is soldered, it is necessary that the plated tin layer have a large weight of 2.8 g/m.sup.2 or more. That is, the plated tin layer should have a large thickness and, therefore, is expensive.
In order to produce cans having a satisfactory quality at a reduced cost, various types of steel materials and welding processes have been remarkably developed. That is, in place of the soldering procedure, various seamwelding processes have been developed, for example, the Soudronic welding process or the Conoweld welding process. In order to enjoy the advantages of the seamwelding processes as much as possible, it is necessary to reduce the amount of the plated tin to a level of 5.6 g/m.sup.2 or less to decrease the cost of the can. However, the reduction in the thickness of the plated tin layer causes the resultant can to exhibit an unsatisfactory resistance to corrosion and, therefore, sometimes, to be practically useless.
Usually, the welding property of the tin-plated steel sheet is very good as long as the amount of the plated tin is 1.12 g per m.sup.2 of each surface of the steel substrate. That is, when a can made from the above-mentioned tin-plated steel sheet is subjected to the Soudronic seamwelding procedure at a welding rate of 50 m/min, substantially no expulsion, surface flash and splash are formed on the welded surface of the can and the resultant weld exhibits a satisfactory strength. Also, a substantially continuous nugget is formed in the weld along the welding direction, and the air-tightness of the resultant seam in the can is satisfactory. However, as stated above, the smaller the amount of the plated tin layer on the steel substrate, the poorer the resistance of the resultant tin-plated steel sheet to corrosion. Therefore, it is desirable to develop a new type of tin-plated steel sheet having a relatively small amount of the plated tin layer and exhibiting a satisfactory resistance to corrosion and an excellent weldability.
The disadvantage of the tin-plated steel sheet having a thin plated tin layer, which has an unsatisfactory resistance to corrosion, can be eliminated by applying a cathodic treatment to the surface of the thin tin layer in a plating bath comprising mainly chromic acid so as to form a coating layer consisting essentially of metallic chromium and hydrated chromium oxide on the tin layer.
Generally, it is known that when the metallic chromium-hydrated chromium oxide coating layer is further coated with a paint layer, the resultant coated steel strip exhibits an excellent bonding property to the paint coating layer and a superior resistance to corrosion under the paint coating layer, to such a great degree that the above-mentioned properties can not be expected of the conventional tin-plated steel strip. The above-mentioned properties are practically provided by conventional steel strips electrolytically treated with a chromic acid solution, for example, CAN SUPER which is a trademark of a chromic acid-treated steel strip made by NIPPON STEEL CORP. JAPAN.
However, the electrolytic chromic acid-treatment applied to the tin-plated steel strip sometimes causes the resultant treated steel strip to exhibit an unsatisfactory seamweldability, for example, in the Soudronic seamwelding procedure.
According to the results of the inventor's research on the welding procedure of the steel strip which has been plated with tin and, then, electrolytically treated with the chromic acid solution, it is known that when the amount of tin, which is free from a tin-iron alloy, is 0.1 g/m.sup.2 or more, preferably, 0.3 g/m.sup.2 or more, the steel strip exhibits a satisfactory seamweldability.
When the seamwelding process is started, the free tin, which is located under the coated chromium layer and which has a relatively low melting point, is melted in the initial stage of the seamwelding procedure. In this stage, the coated chromium layer and the plated tin layer are pressed by an electrode wire of the seamwelding apparatus. Therefore, the coated chromium layer and the plated tin layer in the weld are discharged to the outside of the weld, so that a satisfactorily seamwelded product is obtained.
Practically, the steel strip which has been plated with tin and, then, treated with the electrolytic chromic acid solution, is coated with paint and, then, heated at a temperature of 190.degree. to 210.degree. C. for 20 to 40 minutes to cure the paint layer. During this heating procedure, the free tin in the plated tin layer alloys with iron in the steel strip substrate to form FeSn.sub.2.
Usually, the heating procedure results in the formation of 0.3 to 0.6 g/m.sup.2 of the Sn-Fe alloy in the plated tin layer, before the heating procedure is applied thereto, is, for example, 0.5 g/m.sup.2, the entire amount of tin in the plated tin layer may be converted to a Sn-Fe alloy by the heating procedure. The Sn-Fe alloy has a high melting point and a lower electroconductivity than that of the free tin. Therefore, when the paint coated steel strip is subjected to the seamwelding procedure at a high speed, the Sn-Fe alloy results in the undesirable formation of expulsion, surface flash and splash.
However, even if the Sn-Fe alloy is contained in the plated tin layer, as long as the amount of the free tin in the plated tin layer is 0.1 g/m.sup.2 or more, the resultant paint coated steel strip exhibits a satisfactory seamweldability. Also, it is known that after a steel strip is electroplated with tin, the resultant tin layer is subjected to a flow treatment to make the appearance of the tin-plated steel strip better. This flow treatment is also effective for alloying the tin with iron. The resultant Sn-Fe alloy has a dense structure and, therefore, is effective for enhancing the resistance of the steel strip to corrosion.
However, if no flow treatment is applied to the tin layer, it is necessary to apply another treatment to the steel strip in order to enhance the resistance of the steel strip to corrosion.